Lab charges extra for cross processing film - reasonable?

My lab charges 2.50 euro for processing slide film (E6 bath).
But for "cross processing" color film (E6 bath) they charge 4 euro.

afaik, they do nothing extra for the buck: in both cases, they do the same processing, only with a different film. Is it reasonable that they charge extra for that? Or are they selling air here?

cross-processing usually makes a chemical mess out of their chemistry, so the extra charge is quite reasonable. I'm surprised it isn't more, frankly, and that they don't have a longer turnaround time (often what labs who are willing to cross-process will do is wait until the end of the day and soup the film before they dump and clean their chemistry.

OK that explains a lot. Thanks.